Effects of radiation on fine vasculature may determine the response of dependent tissues late after a course of radiotherapy. Additional stress to irradiated tissues may cause additional injury to fine vasculature causing degeneration leading to acute necrosis. The primary objective of this study is to determine the capability of endothelial cells to repair radiation damage. Removal of layers of the cornea of the eye will cause rapid proliferation of capillary endothelial cells at the limbus. Irradiation of the cornea with sufficiently high doses will prevent endothelial proliferation. The dose required to prevent neovascularization of the cornea in 50 percent of eyes (NVD50) of dogs is approximately 1000 rads. The proposed experiments are designed to determine endothelial cell capability for repair by utilizing split dose techniques. The increase in total dose required to produce a given effect when given as two doses compared to a single dose will indicate this capability. Irradiations done prior to surgery will provide data on undisturbed, slowly proliferating endothelial cells. Irradiation 24 hours following surgery will provide data for rapidly proliferating endothelial cells. These comparative studies of slowly and rapidly proliferating cells may provide insight into response of other slowly proliferating tissues. This model system will also be useful for studying comparative effects of negative pi mesons. Of concern is the response of normal tissues to negative pi mesons in the region of peak energy distribution. Presumably tissues would have little capability for repair of damage produced in that region.